Why Can’t a Transformer Be Operated on DC Supply?

Electrical Technology

What Happens When the Primary of a Transformer is Connected to the DC Supply?

A transformer is a device which step-up or step-down the level of AC current or voltage without changing the primary (i.e. input source) frequency.

Transformer only works on AC and can’t be operated on DC i.e. it has been designed to be operated only and only on alternating current and voltage. To know that what will happen if we connected a DC source to the primary of a transformer, see the following examples where we connect a transformer to the AC first and DC then.

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Transformer Connected to the AC Supply

Suppose we connect a transformer to the AC supply with the following data.

Primary Voltage = V₁ = 230V
Resistance = R₁ = 10 Ω
Inductance = L = 0.4 H
Source Frequency = 50Hz

Lets see how much current will flow through the primary of a transformer in case of AC.

We know that resistance in AC = Impedance

Impedance = Z = V / I in Ω

Where Z = √ (R² + X_L)² in case of inductive circuit.

X_L= 2πfL

X_L= 2 x 3.1415 x 50Hz x 0.4H

X_L= 125.67Ω

Now for impedance

Z = √ (R² + X_L)²

Putting the values

Z = √ (10² Ω + 125.67²Ω)

Z = 126.1 Ω

Now current in the primary

I = V / Z

I = 230V / 126.1Ω = 1.82A

The Primary current in case of AC = 1.82A

Transformer Connected to the DC Supply

Now connect the same transformer to the DC voltage and lets see what happens.

We know that there is no frequency in DC i.e. f = 0. Therefore, the inductive reactance X_Lwould be zero if we put f = 0 in the X_L= 2πfL.

Thus, current in the primary of a transformer in case of DC source.

I = V / R

I = 230V / 10Ω

I = 23A.

The Primary current in case of DC = 23A

The above calculation shows that excessive current will flow in the primary of a transformer in case of DC supply which will burn the primary coils of the transformer. This is not the only reason as the current will be DC, now lets see what happens in case of stady state current in the transformer.

If the primary of a transformer is connected to the DC supply, the primary will draw a steady current and hence produce a constant flux. Consequently, no back EMF will be produced. They primary winding will draw excessive current due to low resistance of the primary because we know that inductive reactance (X_L) is zero due to the inductive reactance formula (X_L = 2πfL) where frequency of the DC source is zero. Thus result is that the primary winding will overheat and burn out or the fuse and circuit breaker will blow. Care must be taken not to the connect the primary of a transformer across the DC Supply.

Why a Transformer Can’t be Operated on DC instead of AC?

If we apply the DC voltage or current to the primary of a transformer, The following are the results

We know that

v = L (di/dt)

Where:

v = Instantaneous voltage across the primary coils
L = Inductance of the inductor
di/dt = Instantaneous rate of current change in A/s

Now in this case, the voltage are constant i.e. DC, Now the current (i) will rapidly increase until the the iron core of the transformer saturated.

At this stage, current (i) will increase to the dangerous level and stop changing. When there is no change in current (i), the induced voltage in the primary will be zero as di/dt = 0 which leads to short circuit the transformer winding with the appalled DC source.

When current exceeded the safe level, high power loss will occur as P = I²R. which will rise the temperature to the dangerous level and there may a chance of explosion of the transformer and transformer oil may catch fire as well.

Or let’s see by Faraday’s Second Law

e = N dΦ / dt

Where

e = Induced EMF
N = the number of turns
dΦ = Change in flux
dt = Change in time

In case of DC voltage to the transformer, there would be constant flux (Φ) induced in the primary due to constant current.

Now the induced EMF in the primary will be zero as (dΦ/dt = 0) i.e. e = N dϕ/dt = 0 due to constant flux induced by constant current.

We also know that there is no frequency in DC supply and flux is inversely propositional to the frequency (Φ = V / f) which saturates the transformer core.

Its mean, the primary of the transformer will act a short circuit path to the additional DC current which may blow the overall transformer. That’s the exact reason we should not connect a transformer to the DC supply instead of AC.

Under What Condition is DC Supply Applied Safely to the Primary of a Transformer?

In most cases, this is an electrical and electronics engineering interview type question, so lets see how to connect a transformer to the DC supply.

There are two Conditions where we can connect a transformer to the DC.

Pulsating DC as Input
High Resistance in Series with primary winding

Pulsating DC in Transformer

In this method, a pulsating DC (which contains ripples and not a pure form of steady state current) to the primary side of a transformer. In this case, the negative cycle reset the flux and time integral of voltage is zero in one complete cycle which again helps to reset the flux in the winding. This concept is used in SMPS (Switched-Mode Power Supply.

High Resistor in Series with Transformer

As we know that a transformer works only on AC. in case of DC supply, the primary of a transformer may start to smoke and burn. But there is a way where we can operate a Transformer on D.C (although the circuit is useless with no output) by adding a high value resistor in series with the primary of the transformer.

When the primary winding of a transformer is to be connected to a DC supply. a high resistance is connected in series with the primary. This series resistance limits the primary current to a safe DC value and thus prevent the primary from burning out.

Please be advised that don’t connect a transformer to the DC supply without a high resistance in series with primary. Because there is no frequency in DC and the impedance (Z) of inductor is zero. If you put Z = 0 in the I = V/Z, Current will be too much high i.e. inductor acts as a short circuit to the DC voltage and currents.